Generally, carbon fibers, which are light in weight and have high strength and a high modulus of elasticity, are in wide use due to their characteristics as a reinforcing material for plastic materials in various application fields such as structural materials for in the aerospace industry, in industrial machines, and in sport and recreational devices. Recently, in particular, high performance carbon fiber having a tensile strength exceeding 600 kgf/mm.sup.2 has been commercialized as a primary structural material for aircraft.
Such high performance carbon fibers are required to have uniform quality in addition to good performance. The carbon fibers used in the aforementioned applications need to be surface treated so as to have an appropriate degree of adhesiveness to a matrix resin. Without the surface treatment, the adhesiveness to the resin would be insufficient, which causes a significant deterioration in the properties of the composite material prepared therefrom due to separation of the fiber from the resin. On the contrary, with excessive surface treatment, the performance of the composite material will frequently be lowered even though adhesiveness to the resin is improved.
Generally, conventional surface treatment processes include oxidation of the surface of the carbon fibers such as a gas phase oxidation treatment with nitrogen dioxide or the like; a liquid phase oxidation treatment with an oxidizing agent such as a perchlorate salt; and electrolytic oxidation treatment using the carbon fiber as an anode.
The electrolytic oxidation treatment using carbon fiber as the anode is industrially advantageous, since high temperature is not necessary in comparison with the gas phase oxidation treatment and a long treatment time is not necessary in comparison with the liquid phase oxidation treatment. This process is disclosed, for example in JP-B-47-40119 (the term "JP-B" as used herein means an "examined Japanese patent publication"), U.S. Pat. No. 3,671,411, etc.
Furthermore, for uniform treatment of the surface of the fiber, there are known processes for applying a uniform current density by selecting the position and the shape of the electrode in an electrolytic bath (JP-A-54-138625, etc.), (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and a process for treating the surface by bringing the fiber sequentially into contact with an anode (a roller) and a cathode (an electrolyte solution) (JP-B-48-12444). (The term "JP-B"
In using an electrolytic bath for surface treatment, a method has been reported where ultrasonic vibration is applied to the electrolyte solution for the purpose of uniformly treating the fiber even in the interior of the fiber bundle (JP-A-62-149970).
Further, for achieving higher performance, certain specific surface treating conditions, particularly, the surface treating energy, should be employed for improving performance as a composite material such as described in JP-A-55-12834.
Other electrolytic surface treatment methods are described, for example, in U.S. Pat. Nos. 3,214,647, 3,759,805, 3,657,082, 3,859,187, 3,671,411, 4,401,533, British Patent 1,326,736, 1,371,621 and 2,018,827A.
During the electrolytic surface treatment, the damage of carbon fiber strands and the formation of fluff should be prevented. For these purposes, there is known a method of flowing an electric current through the carbon fiber using an electrolyte solution without contact with an electrode roller or a guide (JP-B-47-29942) and a method employing an electrolyte solution overflowing from an anode solution bath and a cathode solution bath a portion of the solution extending above the container of the solution due to the surface tension of the solution.
For the effective industrial surface treatment of carbon fiber, the apparatus therefor is necessarily large and complicated in order to treat a large number of strands uniformly at one time without quality impairment such as fluff generation. In any of the above methods, surface treatment baths are employed, which result in bubbles of air, hydrogen or the like attaching to the surface of the carbon fiber while a fiber strand is passing through the bath, which tends to cause variations in the surface treatment, and which also requires any circulating solution which is used to be increased in quantity. To achieve higher productivity, variations in the surface treatment achieved in the breadth direction are liable to be caused as a result of the scale-up of the apparatus, and variations in the length direction are liable to be caused by an increased treating bath length. No method has been found for solving such problems. The present invention intends to solve the above-mentioned problems.